Various implantable medical devices are advantageously inserted within various body vessels, for example from an implantation catheter. Minimally invasive techniques and instruments for placement of intraluminal medical devices have been developed to treat and repair such undesirable conditions within body vessels, including treatment of venous valve insufficiency. Intraluminal medical devices can be deployed in a vessel at a point of treatment, the delivery device withdrawn from the vessel, and the medical device retained within the vessel to provide sustained improvement in vascular valve function. For example, implantable medical devices can function as a replacement venous valve, or restore native venous valve function by bringing incompetent valve leaflets into closer proximity. Such devices can comprise an expandable frame configured for implantation in the lumen of a body vessel, such as a vein. Venous valve devices can further comprise features that provide a valve function, such as opposable leaflets.
Implantable medical devices can comprise frames that are highly compliant, and therefore able to conform to both the shape of the lumen of a body vessel as well as respond to changes in the body vessel shape. Dynamic fluctuations in the shape of the lumen of a body vessel pose challenges to the design of implantable devices that conform to the interior shape of the body vessel. For instance, the flow velocity and diameter of veins do not remain essentially constant at a given systemic vascular resistance. Instead, the shape of vein lumens can fluctuate dynamically in response to the respiration, body position, central venous pressure, arterial inflow and calf muscle pump action of a mammalian subject. The veins also provide the principal volume capacitance organ. For example, an increase of almost 100% in the diameter of the common femoral vein has been observed in human patients simply by rotation of the patient by about 40 degrees, corresponding to a four-fold increase in blood flow volume. Moneta et al., “Duplex ultrasound assessment of venous diameters, peak velocities and flow patterns,” J. Vasc. Surg. 8; 286-291 (1988). The shape of a lumen of a vein can undergo dramatic dynamic change as a result of varying blood flow velocities and volumes therethrough, presenting challenges for designing implantable intraluminal prosthetic devices that are compliant to the changing shape of the vein lumen.
However, an implantable medical device comprising a highly compliant frame can present other drawbacks in some applications, such as for providing a support structure for remodelable material. For treatment of many conditions, it is desirable that implantable medical devices comprise remodelable material. Implanted remodelable material provides a matrix or support for the growth of new tissue thereon, and remodelable material is resorbed into the body in which the device is implanted. Common events during this remodeling process include: widespread neovascularization, proliferation of granulation mesenchymal cells, biodegradation/resorption of implanted remodelable material, and absence of immune rejection. By this process, autologous cells from the body can replace the remodelable portions of the medical device.
Mechanical loading of remodelable material during the remodeling process has been shown to advantageously influence the remodeling process. For example, the remodeling process of one type of remodelable material, extracellular matrix (ECM), is more effective when the material is subject to certain types and ranges of mechanical loading during the remodeling process. See, e.g., M. Chiquet, “Regulation of extracellular matrix gene expression by pressure,” Matrix Biol. 18(5), 417-426 (October 1999). Mechanical forces on a remodelable material during the remodeling process can affect processes such as signal transduction, gene expression and contact guidance of cells. See, e.g., V C Mudera et al., “Molecular responses of human dermal fibroblasts to dual cues: contact guidance and mechanical load,” Cell Motil. Cytoskeleton, 45(1):1-9 (June 2000).
A variety of remodelable materials are available for use in implantable medical devices. For instance, naturally derived or synthetic collagenous materials can be used to provide remodelable surfaces on implantable medical devices. Naturally derived or synthetic collagenous material, such as extracellular matrix material, are another category of remodelable materials that include, for instance, submucosa, renal capsule membrane, dura mater, pericardium, serosa, and peritoneum or basement membrane materials. One specific example of an extracellular matrix material is small intestine submucosa (SIS). When implanted, SIS can undergo remodeling and induces the growth of endogenous tissues upon implantation into a host. SIS has been used successfully in vascular grafts, urinary bladder and hernia repair, replacement and repair of tendons and ligaments, and dermal grafts.
Presently, medical devices often comprise frames with a fixed degree of compliance that does not change over time. Therefore, optimizing the degree to which a medical device for implantation within a body vessel is compliant to changes in the shape of the body vessel can present a trade-off between competing factors. For example, a medical device comprising a highly compliant frame can minimize distortion of a body vessel by being highly responsive to changes in the shape of the body vessel. However, a frame with less compliance may provide inadequate mechanical loading to material attached to the frame to allow or promote certain desirable processes to occur within the attached material, such as remodeling, or within the body vessel. In this example, frame compliance is a trade-off between enabling the remodeling of material attached to the frame, and minimizing the distortion or disruption of the body vessel.
What is needed are medical devices that provide changing compliance over time. There exists a need in the art for an implantable prosthetic device frame that is capable of balancing concerns of conforming to the shape of a body vessel lumen and providing optimal tension on a remodelable material attached to the frame.
By providing frames with compliance that can vary with time, embodiments of the present invention enable one skilled in the art to design, make and use medical devices that provide desired levels of compliance at different time periods. Medical devices with variable compliance can provide, for example, an optimal amount of tension on an attached remodelable material during the remodeling process, and then provide increased compliance and minimal body vessel distortion after the remodeling process is completed.